Recovery of metal values from clays, ores and concentrates by acid leaching has widespread use. The acid leaching of these materials generally results in an acidic solution containing the metal to be recovered together with acid-soluble metallic impurities, mainly iron. Unless these acid-soluble metallic impurities are separated from the desired metal, the final product will be contaminated and unsatisfactory for its intended use. A common method employed for the removal of dissolved iron impurities from aqueous, acidic leach solutions is the use of an organic extractant which selectively extracts from the leach solution the iron impurity, usually in the form of a complex. In order to allow the recycle of the organic extractant for further purification purposes the iron impurity containing organic phase is generally stripped with an aqueous acidic solution. Stripping will destroy the complex and the iron impurity will then be dissolved in the acidic stripping solution. In commercial scale operations large volumes of acidic stripping solutions are generated and the disposal of these solutions presents a problem from both an environmental point of view and from a purely technical and economical standpoint. Due to its acidity the solution cannot be directly disposed of into natural receiving bodies, neutralization is costly and it may generate further problems for example disposal of solid waste.
It has already been suggested to remove the dissolved metal salts from the stripping solutions by crystallization and subsequent separation of the crystallized salt from the residual liquor. Crystallization involves extended time periods and/or requires large pond areas to provide the required surface, particularly in view of the large volumes involved. Acceleration of the crystallization process by using evaporation at elevated temperatures requires significant external energy and care, for example in case of hydrochloric or nitric acid solutions, pose additional problems, such as generation of envionmentally undesirable acid fumes.
It has now been discovered that the iron salt content of iron-containing acidic stripping solutions can be readily removed in the form of solid iron oxide without requiring neutralization and encountering environmental problems by hydrothermally treating the acidic solution at elevated temperatures and pressures. This treatment causes the conversion of dissolved iron sulfate, nitrate or chloride to solid iron oxide which can be separated from the residual liquor. The residual, substantially iron-free acid solution can be, if desired, recycled to the stripping of iron salt laden organic extractants.